PHOTOGENERATED CHARGE STORAGE IN HETERO LANGMUIR-BLODGETT-FILMS - ELECTRON-TRANSFER MECHANISM

Photoinduced electron transfer (charge separation (CS)) and back electron transfer (charge recombination (CR)) were investigated in the temperature range 7-320 K for the following acceptor-photosensitizer donor hetero Langmuir-Blodgett film device: indium tin oxide/insulator layer (185 angstrom)/two layers of steroidal tetracyano-p-quinodimethane (acceptor) (36 angstrom)/two layers of a Cu phthalocyanine derivative (photosensitizer (S)) (28 angstrom)/two layers of steroidal p-phenylenediamine (donor) (48 angstrom)/insulator layer (380 angstrom)/Al. The transient photocurrent in the CS process did not change very much in the temperature range observed. The apparent first-order rate constant k(CR), obtained from the differential of the film voltage decay curve, decreased as CR proceeded. Arrhenius plots of k(CR), at various CR probabilities P indicated that both the thermal activation of reactant oscillators and the quantum-mechanical nuclear tunneling process existed, and that the former was predominant at high temperatures. The in-plane amorphous structures of monomolecular dye films can be considered to cause an activation energy distribution, which became wider as the temperature decreased, and also to cause a wide tunneling rate distribution. An irreversible decrement in the initial photovoltage occurred at 320 K, probably owing to glass transitions of the acceptor and insulator films. Similarities to electron transfer behaviors in natural photosynthetic reaction centers are also discussed.